BUILD_BROKEN_DUP_SYSPROP as escape hatch for the new sysprop restriction

As the final step for the refactoring of sysprop configuration, this
change adds BUILD_BROKEN_DUP_SYSPROP which is the escape hatch for
the new restriction. When it is turned on, the new syntax `a ?= b`
collapses to the old syntax `a = b`, duplicated assignments are allowed,
and the dups are resolved following the legacy rule of preferring the
first.

This change also summarizes all the user-facing changes to the Change.md
file.

Lastly, post_process_prop.py is refactored to accept new argument
'--allow-dup' which when turned on allowes duplicated sysprops.

Bug: 117892318
Bug: 158735147
Test: atest --host post_process_prop_unittest
Change-Id: I7bdfffd47d50aad66a78e28a30c3dad7ebac080c